1. Field of the Invention
The present invention relates to a radio receiving method and a radio receiving apparatus which are used to obtain baseband signals by orthogonally converting radio receipt signals after being frequency-converted, when receiving and demodulating a simultaneous paging signal such as a pager, etc.
2. Description of the Related Art
Conventionally, a pager has been known as one of radio receiving apparatus which receive a simultaneous paging signal, telephone number and business of a transmission pager (caller) and display the same. A super heterodyne system has been employed for conventional pagers. In the super heterodyne type pagers, a narrow-band filter, for example, a band-pass filter (BPF) is disposed between an antenna and a receiving amplifier. However, since the BPF is large-sized, there is such a problem where if a BPF is equipped in a pager, the pager itself is unavoidably made large in size. Therefore, in recent pagers, a direct conversion (DC) receiving system which has a simple construction and is able to be small-sized is employed.
FIG. 8 is a block diagram showing major parts of a DC receiving type radio receiving apparatus. FIG. 8 shows a frequency signal processing system to orthogonal conversion in a radio receiving apparatus in which a DC receiving system is employed. In the radio receiving apparatus, radio waves of FSK system coming from the base station are received by antenna Antl, and the receiving signal (f0) is amplified by a high-frequency amplifier 2. Amplification signal (f0) coming from high-frequency amplifier 2 is inputted into mixers 3a, 3b. Local oscillation signals (cosine wave signals, sine wave signals) which are of the same frequency (fL) as that of the receiving signals (f0) are inputted into these mixers 3a, 3b, wherein the receiving signals (f0) are multiplied by the local oscillation signals.
A local oscillation signal (fL) of cosine wave signals outputted from local oscillator 7 is supplied into the mixer 3a, and sine wave signals, in which the phase of the local oscillation signals (fL) outputted from the local oscillator 7 is shifted 90.degree. by phase shifter 8, is supplied into the mixer 3b. Orthogonal conversion is carried out by multiplying the receiving signal (f0) by local oscillation signals (cosine wave signals, sine wave signals) in the mixers 3a, 3b, whereby base band signals (I component signals and Q component signals) are obtained.
I signals and Q signals outputted from mixers 3a, 3b are inputted into a demodulation circuit 6 after they are band-limited through channel filters 4a, 4b having the center frequency corresponding to the receiving channel (ch) and limiters 5a, 5b. Then, demodulation signals are outputted from the demodulation circuit.
Furthermore, at the local oscillator 7, oscillation circuit 9 is caused to oscillate at the reference frequency of crystal oscillator XTAL, wherein oscillation signals of oscillation circuit 9 are given to phase locked loop (PLL) portion 10 consisting of a phase comparator, prescaler, programmable divider (counter), mixer, etc. (which are not illustrated). The oscillation frequency of voltage control oscillator (VCO) 12 is locked to a specified frequency by a control voltage which is generated by causing the output signals of PLL portion 10 to pass through a low pass filter (LPF) 11. The oscillation signal (fV), the frequency of which is locked, is multiplied by multiplication circuit 13, and local oscillation signals having the same frequency (fL) as that of the receiving frequency (f0) are sent out to mixers 3a, 3b. In the PLL portion 10, programmable divider, etc. receives control signals from CPU 14 to cause the receiving frequency to be established.
The entirety of local oscillator 7 is covered by shield case 15, whereby unnecessary radiation is attempted to be decreased. However, especially, since the resonance frequency of antenna Ant1 is identical to that of local oscillation signals, the local oscillation signals are radiated through antenna Ant1. This becomes unnecessary radiation waves and is received again. If a phase difference exists between the unnecessary radiation waves and the receiving frequency (f0), noise is generated in the demodulation signals. Furthermore, unnecessary radiation of intensive electric field strength is mixed from a circuit substrate member, at which local oscillator 7 cannot be shielded by a shield case 15, etc., into a mixer.
Improvement of unnecessary radiation from an antenna in such a DC receiving system was proposed. For example, as disclosed in Japanese Laid-open Patent Publication No. 321686 of 1995, local oscillation signals inputted into a mixer where orthogonal conversion is carried out are set in a phase relationship where they are drowned each other. Thereby, unnecessary radiation power from antenna can be decreased.
However, in the example of the abovementioned Patent Publication, although improvement of decreasing unnecessary radiation power from antenna was attempted, it is difficult to carry out an accurate orthogonal conversion by the mixer, and simultaneously the apparatus will be made large-sized, wherein the actual mounting area of electronic components is decreased, and the process work will be made cumbersome.
That is, since local oscillation signals coming from the local oscillation portion are, directly or through a common earth, jumped into a mixer or antenna system, it is necessary to cover the local oscillation portion with a shield case as in the description of the former in the conventional example. Furthermore, since local oscillation signals are jumped directly into the antenna system, it is difficult to securely prevent local oscillation signals from being radiated from an antenna, the frequency of which is the same as that of the local oscillation signals. Therefore, unnecessary radiation waves are received again, wherein as a phase difference occurs between the unnecessary radiation waves and the receiving frequency, noise is caused to occur in the demodulation signals.
Furthermore, If the local oscillation portion is covered by a metal-made shield case, space is caused to occur between members inside the shield case and internal electronic components. In other words, the shield case is made large-sized. Resultantly, this becomes an obstacle in view of requirements of downsizing and light weight.
Still furthermore, since a large-sized shield case is disposed, the actual mounting area of secondary and thirdly circuit substrates of electronic components is decreased. In this case, since more electronic components can not be mounted, no high performance and downsizing can be achieved. Furthermore, since it is difficult to automatically dispose a large-sized shield case in the standard automatic surface mounting machine, manual work is requisite. That is, a shield case attaching process and soldering process are made cumbersome, wherein the assembling efficiency is worsened.